Clean up start-containers.sh

[Mierdin]

If launched with no args, getting:

scripts/start-containers.sh: line 97: [: 0: unary operator expected

This is indicative of a lack of clear direction on how to launch this script. There is also not a clear direction on when the image is pulled from docker hub or built locally.

The args should be standardized and checked before the rest of the logic runs. And it should be clearer if the image is going to be downloaded or built.

Should also add to docs how to use it